1. Field of the Invention
The subject application generally relates to a method of determining an amount of impurities that a contaminating material contributes to a high purity silicon that is heated in the presence of the material, and is further directed to furnace that minimizes the amount of impurities that are contributed from the furnace to the high purity silicon.
2. Description of the Related Art
Methods of processing high purity compositions are known in the art, especially in the semiconductor industry where it is desirable to manufacture high purity crystalline silicons. The amount of impurities in the high purity crystalline silicons directly correlates to performance of the high purity crystalline silicons in semiconductors. Therefore, there is a constant desire to minimize impurity content of the high purity crystalline silicons, as well as a general desire to minimize impurity content in any high purity composition.
Extreme measures are generally taken to minimize the amount of impurities in high purity compositions, such as high purity crystalline silicons, including processing the high purity compositions in clean rooms under atmospheric isolation. Furthermore, personnel working in the clean rooms typically wear protective suits to prevent fibers from clothing or other chemicals that may be introduced into the clean room from the personnel from contaminating the clean environment.
When manufacturing high purity crystalline silicons, in particular, a chemical vapor deposition (CVD) process is generally used to grow polycrystalline silicon on a silicon slim rod from chlorosilane gas, thereby forming a polycrystalline silicon log. After the CVD process, the polycrystalline silicon log is subjected to a zoning process whereby the polycrystalline silicon is converted into single crystalline silicon. As known in the art, impurities present in the polycrystalline silicon log are removed through the zoning process. Alternatively, the polycrystalline silicon log may be used to make silicon slim rods, which are subsequently used to create more polycrystalline silicon logs through the CVD process. To make the silicon slim rods from the polycrystalline silicon logs, the logs must be cut. However, the polycrystalline silicon logs are brittle and must be annealed to reduce internal stresses, which enables the polycrystalline silicon logs to be effectively cut into silicon slim rods without breaking.
To anneal the polycrystalline silicon logs, the logs are placed in a furnace and heated at a sufficient temperature for a sufficient time to anneal the logs. However, annealing contributes impurities to the polycrystalline silicon logs, which is undesirable. More specifically, environmental impurities, such as dust or other molecules present in the ambient atmosphere, may contaminate the polycrystalline silicon logs during annealing. Furthermore, materials that are actually used to form the furnace and that are heated during annealing release impurities contained therein. The impurities released by the materials that are used to form the furnace during annealing are subsequently absorbed by the polycrystalline silicon logs during annealing.
While it is known that impurities are contributed to the polycrystalline silicon logs during annealing, little focus has heretofore been given to reducing the amount of impurities contributed to the polycrystalline silicon logs during annealing. To explain, only polycrystalline silicon logs that are cut into silicon slim rods are generally subject to annealing. Because silicon slim rods account for only a small fraction of the overall volume of the resulting polycrystalline silicon logs made therefrom (typically about 0.6% by volume), the overall amount of impurities in the polycrystalline silicon logs is minimally impacted by the impurities contributed to the silicon slim rods through annealing of polycrystalline silicon logs that are cut to make the silicon slim rods. As such, efforts to minimize the amount of impurities in the polycrystalline silicon logs generally focus on other sources of impurities other than the furnaces used to anneal the polycrystalline silicon logs. However, in the drive to constantly reduce the amount of impurities present in high purity compositions, such as high purity crystalline silicons, there remains a need to reduce the amount of impurities that are contributed to the high purity compositions at all stages of processing, including during annealing, and to provide furnace assemblies that minimize dust generation due to movement of various components of the furnace assemblies or due to movement necessary to introduce the high purity compositions into the furnace.